First match point saved. Wawrinka opts to serve and volley on his second serve, but it does not work as Federer passes with ease.

Federer forces the error from Wawrinka, who pulls a forehand wide for 30-30, but Federer hits the top of the net for 40-30. Match point for Wawrinka.

Warinka wins the first point, but loses the second. 15-15, then 30-15. Two points for victory.

Leon Smith
GB Davis Cup captain on BBC Radio 5 live sports extra

"It's going to be interesting to see how the crowd support each player when Wawrinka comes out to serve for the match. Do they try and raise Roger Federer? What a moment this is for Stan. He's nerves will be tested against his friend."

Federer keeps it interesting by holding his serve. There had been a delightful rally with both players volleying while close at the net, but Federer eventually maintains the pressure on his opponent. But Wawrinka is now serving for the match. Can he hold his serve and nerve?

Wawrinka has never beaten Federer on any surface apart from clay.

Leon Smith
GB Davis Cup captain on BBC Radio 5 live sports extra

"I think we'll see Wawrinka try everything and anything to break here because it will be uncomfortable trying to serve out for the match."

A break point for Federer after an epic rally. He just does not know when he is beaten. But he drags a cross-court forehand wide of the target. not by much, but it is out. He looks to the heavens as that was a great chance, but instead we are at deuce.

Federer than has another break point with a powerful smash. Wawrinka is reduced to throwing his racquet at the high bouncing ball. But Wawrinka for the second time saves the danger, before winning the game.

The world number two needs perfection from now on. He is serving to stay in the match.

David Law
BBC Radio 5 live sports extra commentator

"Federer has not been sure what tactic to use. He's tried lots of different things and has just about stayed in the match but Wawrinka has been the better player."

Image source, Getty Images

Roger Federer is still chipping away at the umpire after the controversy at the start of the set. Let it go, Roger. Let it go.

David Law
BBC Radio 5 live sports extra commentator

"You wonder if Wawrinka is starting to think about the position he is in."

The frustration is getting to Federer. He loses a long rally. Once the point is over, he smashes a bouncing ball into the stand. The six-time season-ending event winner is under pressure in what is now the longest match of the week.

But like all champions, he shows his fighting spirit by recovering and forcing Wawrinka to go long with his forehand. Advantage Federer then becomes game Federer. An absolutely fantastic match.

Leon Smith
GB Davis Cup captain on BBC Radio 5 live sports extra

"Federer was deep in thought after he missed that last shot. He's probably trying to steady himself but one thing is for sure he won't give this match up."

A rare error from Federer, and it could prove costly. An overhead smash, close to the net, goes wrong and that helps Wawrinka take his service game, without the world number two winning a point.

Wawrinka has only beaten Federer twice in 16 matches. He needs two more games for his third victory.

Another of your Facebook questions, external to former British number one Tim Henman.

Jennifer Wyatt: "As a tennis fan, who is your favourite player, and why?"

Image source, Getty Images

TH: "I've probably got three of all time. Bjorn Borg was my favourite when I first got interested in the game. Stefan Edberg was then a hero of mine when I started playing professionally - I was lucky enough to practice a lot with him because he was living in London. Now my favourite player to watch is Roger Federer."

Leon Smith
GB Davis Cup captain on BBC Radio 5 live sports extra

"Huge hold for Federer. How important that second serve was at 30-30. He went out wide when most people would have gone for a safer option. That's why he's a champion."

But nothing is going to be handed to Wawrinka. His Davis Cup team-mate is doing everything he can to stay in the match. Federer again holds his serve. The deficit is again reduced to one game.

Wawrinka does enough again. He is managing to do enough to hold onto his lead, and once again his serving his been emphatic. He restores his two-game lead and inches closer to a place in the final.

He lost to Novak Djokovic in the semi-finals last year at the O2 Arena. He may well be getting his chance for revenge.

Just what Federer needs to regain his composure after the earlier controversy - a routine hold of serve. He is still a break down in the third, but at least he looks a bit less angry than he did five minutes ago.

Image source, AFP

An international footballer in the crowd for you to guess. Full marks for those who recognised Real Madrid's James Rodriguez watching on.

Federer's mood gets even worse when Wawrinka holds his serve in the second game. Suddenly Federer is looking vulnerable.